Protocol Error modify Exchange LDAP serv

Hi,
I'm trying to add entries into an MSExchange (v5.5) LDAP server using ldapmodify -a. However, every time I attempt to do this I get an "ldap_add: Protocol error" message. Not only that but I don't appear to even have access to the whole directory server. I get the Protocol error mesage at the cn=services, cn=Recipients, ou=TMCSCATS, o=RTA level, and an Insufficient Access error at any other level. Has anyone any ideas?
Thanks,
Aoife